The invention relates to a display device comprising a first substrate having a matrix of ferro-electric pixels arranged in rows and columns and comprising a ferro-electric liquid crystal material with a deformable helix between the first and a second substrate and comprising a group of row or selection electrodes and a group of column or data electrodes, each pixel on at least a first substrate comprising a picture electrode which is connected to a column electrode or row electrode via an active switching element, the display device comprising means for presenting selection voltages to the row electrodes and data voltages to the column electrodes and for bringing, prior to selection, a row of pixels to a fixed optical transmission state by means of an auxiliary signal during at least one of two consecutive drive periods.
Such display devices are applicable as video displays, but also, for example in datagraphic monitors or as viewfinders.
A ferro-electric liquid crystal material with a deformed helix is usually understood to mean a ferro-electric liquid crystal material having a natural helix whose pitch is smaller than the wavelength of visible light (up to approximately 400 nm). An electric field perpendicular to the axis of the helix deforms this helix, which results in a rotation of the optical axis. The transmission between crossed polarizers, with one of the polarizers being parallel to the axis of the helix, then increases with the value of the field for both positive and negative values of the field.
A display device as mentioned above is described in "A Full-Colour DHF-AMLCD with Wide Viewing Angle" in SID 94 DIGEST, pp. 430-433. The use of devices with DHFLC material (Deformed Helix Ferro-electric Liquid Crystal) is described in this article as being advantageous with respect to SSFLC devices (Surface Stabilized Ferro-electric Liquid Crystal) due to the absence of multidomains, while due to a more continuous change of the transmission/voltage characteristic grey levels can be better realised. In spite of the rapid switching time which is mentioned for the mixture used in the display device, the frame frequency remains, however, too low for video applications (NTSC or PAL). In the device described a phenomenon referred to as "image sticking" or "after images" also occurs.